Written by Ada Szczepaniec, former SDSU Extension Entomology Specialist.

Dry weather in the Midwest has already brought spider mite woes to Indiana, Ohio, Iowa, and Nebraska, and we are getting reports of spider mites in South Dakota as well. Wet and humid weather promotes growth of pathogens that attack spider mites, so in wet years mites are unlikely to reach high numbers. Spider mites are not insects; they are arachnids and belong to a large family of mites (Tetranychidae) that earned their common name because many of them produce webbing when their populations are high. Spider mites feed on plants by piercing the plant tissue and sucking up contents of plant cells causing characteristic stippling damage (small spots). If their numbers are high, spider mites cause leaves to turn yellow and drop from the plants. Spider mites can go through many generations each season, and in hot and dry conditions they can take as little as 10 days to complete development. There are several key predators of spider mites that keep their populations in check such as predatory mites, spider mite destroyers (small beetles from the same beetle family as ladybird beetles), and predatory thrips.

Twospotted spider mites can be a problem in soybeans in dry summers, but there are no hard and fast economic thresholds for spider mites in soybeans. Spider mite treatment guidelines for soybeans are based more on observable plant damage than on counts, as the mites are hard to see. Spider mite damage usually starts in the lower canopy and progresses to the middle and upper canopy as populations build. A good rule of thumb is to treat when stippling reaches the middle canopy.

Spider mite damage may resemble symptoms of drought stress or certain diseases so ensure that spider mites are present on the leaves before making decisions about pesticide treatments. Using a 10X hand lens is usually necessary to see the mites. A quick way to isolate them is to tap potentially infested foliage over black construction paper and examine the “grit” with your lens. Many pyrethroid insecticides can flare up spider mites further by repelling but not killing the mites, and eliminating their natural predators. Using an insecticide such as dimethoate may be a better option than pyrethroids.

Above: Spider mites feed on the undersides of leaves and cause a stippling damage visible on the upper sides of leaves.

Photo: Galen Dively, University of Maryland

Above: Large infestations of spider mites cause the leaves to turn brown and drop from the plants.

Photo: Galen Dively, University of Maryland

Twospotted spider mites and another spider mite species, Banks grass mites, can also be a significant pest in corn fields. Banks grass mites look similar to twospotted spider mites but the two dark areas run along the length of their sides, and do not end in the middle of their bodies. Research in Texas has demonstrated that both species of spider mites have similar potential to damage corn so the same thresholds should be used in decision-making process for both twospotted and Banks grass mites. Both species of spider mites reproduce fast, have multiple generations per season, and their generations times are very short. Spider mites can complete their development in as little as 10 days in dry and hot conditions.

Above: Two spotted spider mite (left) and Banks grass mite (right). Banks grass mites have dark green areas that run along the length of their bodies.

Image: University of Nebraska, Lincoln, Extension.

Above: An example of spider mite damage to corn.

Photo: A. Szczepaniec

To scout for spider mites, inspect at least 10 plants throughout the field. Make note of the proportion of green corn leaves that are infested with spider mites on each plant, and record the percentage of leaf area on each infested leaf that is damaged by spider mites. Symptoms of spider mite damage to leaves include chlorotic (white to yellow) spots on plant tissue where spider mites sucked out the contents of plant cells. We tend to overestimate percent damage usually, so be conservative in estimates of percent area that is damaged. Use the table below to evaluate if pesticide applications are recommended based the cost of treatment given the market value of the crop. Alternatively, use guidelines established by the extension specialists in Colorado, who recommend treating if damage is visible in the lower third of the plant, and spider mite colonies are visible in the middle third of the plant. Because eggs of spider mites are not killed by pesticide applications, scouting for spider mites should be repeated after treatments. Before the initial pesticide applications, clearly mark 25 leaves infested with spider mites throughout the field, and inspect those leaves after pesticides have been applied and the field re-entry period has passed. If there are mobile stages of mites present on those marked leaves approximately 5-7 days after the initial treatments, additional applications may be necessary. It is important to note that according to research done in Texas and Colorado, corn yield is unlikely to benefit from pesticide applications to manage spider mites after the dent stage. Again, once corn is in the hard-dough stage, there is no benefit from treating for spider mites.

Adhering to thresholds is important because there are many natural predators of spiders mites that are effective at keeping their populations in check. Applying pesticides before it is necessary will annihilate natural predators and further exacerbate potential for spider mite outbreaks. We are providing a table with pesticides registered for spider mite control in corn and soybeans.

Table 1: Click for larger image.

Economic injury level for the spider mites attacking corn. Pesticide applications should be considered when economic injury levels are reached. Source: Texas A&M University, Extension Publication E-400. This table is for corn values at or lower than $700 per acre, and Bob Wright, University of Nebraska Extension specialist provided the following for market values that exceed $700 per acre:
Use the following formulas to determine an economic injury level.

For percent infested leaves the formula is:

(cost of control x 600) ÷ (price per bushel x bushel yield)

For percent of leaf area damaged the formula is:

(cost of control x 312) ÷ (price per bushel x bushel yield)

Above: Sethorus punctillum, a cocconellid beetle from the same family as ladybird beetles, commonly known as spider mite destroyer.

Above: Predatory thrips (the smaller, long insect in the image, almost see-through) consuming a spider mite female

Above: Nymph of Orius insidiosus, a hemipteran generalist predator. Their common name is minute pirate bugs.

Above: Lacewing larva (Chrysoperla sp.).

Above: Nymph of Orius insidiosus, a hemipteran generalist predator. Their common name is minute pirate bugs.